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Synchronous Machines

A. Preparation

B. Demonstration

C. Report

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B. DEMONSTRATION

In this exercise we will take data sufficient to plot several Vee-curves for a synchronous motor. We will also take data that will allow inference of the machine reactance and the approximate relation of machine induced voltage to field current. From this latter information, you are to calculate and plot theoretical Vee-curves for the synchronous motor and compare them with the experimental ones.

1. For Ifield = 0.0 (0.5) 3.5 A in the synchronous machine, rotate it with a DC motor and measure the open circuit output voltage (E) of the synchronous machine at its synchronous speed (1200 rpm). You may also wish to vary the excitation of the DC machine to see its effect on rotor speed. Be sure to conclude with a measurement of open circuit voltage with Ifield slowly lowered to 0.0 A.

2. With Ifield = 0.0 Amp and the field poles demagnetized, apply 175 V (line-line, 3-phase, 60 Hz), to the AC machine and measure and record line current, phase voltage, and phase power. This allows calculation of the machine’s equivalent reactance.

3. Connect the DC machine as a separately excited DC generator driving a load of 6 ohms. Set the field current of the AC machine initially to 0.5A. Start the AC machine by bringing the supply voltage up to 208 V (line-line). Quickly increase the field current so that it runs as a synchronous motor and adjust the field current to approximately 1.5 Amps. Next, insert a DC generator field current (close switch) and vary the field current as needed to change the DC generator voltage and load power and, therefore, the AC machine load to obtain a phase power PA = 200 W. Be sure the phase voltage VA is 120 V. Now vary the AC machine field current Ifield 0.0 (0.5) 3.5A to obtain a curve of IA versus Ifield for PA = 200 W. Adjust the DC machine field current as required to maintain PA = 200 W. Repeat for PA = 450 W and 700 W. Note that at each power setting, VA should be reset to 120 V at an Ifield setting yielding near minimum IA. For each PA and Ifield setting, record input power PA, line current IA, and phase voltage VA, and field current Ifield. This data allows plotting experimental Vee-curves (IA versus Ifield at constant PA) for the motor. Also, near each minimum value of IA, record DC generator voltage VDC and load current IDC. This data allows calculation of the efficiency of this AC to DC power converter system.

C. REPORT

SPECIAL INSTRUCTION: For each step below, include tabular presentations of the raw experimental data and the reduced or calculated data as required.

1. Using the data obtained in B.1, make a plot of the E values versus Ifield and obtain a smooth best fit curve for these data points. Comment on these results. (Note that E = EA.)

2. Using the data obtained in B.2, calculate X. Show your calculation in detail. Discuss the effect of any uncertainty in ( on the calculated value of X.

3. (a) Using the data obtained in B.3, plot on the same chart the experimental Vee-curves showing armature current (I) versus field current (If) for each of the input power settings. (Note that I = IA = IX and If = Ifield.)

b) Examine each of these Vee-curves and their corresponding data and determine for each power P the minimum value of armature current (I) and the corresponding field current (Ifm) for each Vee-curve. Make a table showing P, I, and Ifm. (Note that P = PA = PX and V = VA.)

c) Determine the experimental (fm = E/V for each value of Ifm using the measured phase voltage (V) and E from the E versus Ifield best fit curve obtained in Step 1 above.

d) Calculate normalized phase power (p) for each P using Equation [F], i.e., p = P/[V2/X]. Make a table showing (fm, V, P, and p.

e) Plot the experimental (fm values as a function of normalized phase power (p) to the synchronous motor for each of the power settings.

f) Finally, calculate the theoretical (min data points computed using Equation [K] and plot on the chart from Part (e) above. Comment cogently on the comparison of (fm and (min.

4. (a) For each value of normalized power p, calculate and plot on the same chart the theoretical Vee-curves for this motor showing i versus (. To do this, first choose a value for Ifield, second, obtain E from the curve fit obtained in Step 1 above, third, calculate ( as E/V, and fourth, calculate i from the Equation [H] relating i to ( and p. Repeat this set of calculations for a sequence of Ifield values for each value of normalized power p. Finally, plot the calculated theoretical data and connect these theoretical (i, () data points with a smooth curve.

b) For each value of normalized power p, create experimental Vee-curve data (i versus (). Note that i is given by the first definition in Equation [G].

(c) Finally, overlay the experimental (i, () points on the corresponding theoretical graphs, but do not connect these points, so that the theory and experiment can be compared.

(d) Comment on the correspondence of the experimental (i, () points to the theoretical Vee- curves. Give possible reasons for the differences between the two.

5 (a) Assuming the three phase synchronous machine presents a balanced load to the three phase power supply, calculate the total three phase power drawn by the synchronous machine for each power setting P when operating at unity power factor (Ifm).

(b) Calculate the DC power delivered to the resistive load for each unity power factor setting (Ifm).

(c) Calculate the total system losses at each power setting

(d) Calculate the efficiency of this AC to DC voltage converter system at each power setting.

(e) Make a table summarizing the total AC power, DC power, system losses, and efficiency for this AC to DC voltage converter setup.

(f) Comment on the results shown in this table.

SYNCHRONOUS MACHINES LABORATORY

STARTING, OPERATING AND SHUTDOWN PROCEDURES

March 12, 2009

Demonstration Step 1 - DC Motor Driving AC Generator

DC Motor Start–up Procedure

1. Insure that both DC voltage supplies are set to the minimum voltage and that they are OFF. Also insure that the AC voltage supply is set to the minimum voltage and that it is OFF.

2. Insure that the switches in the field circuits of the two machines are both OFF.

3. Connect the field circuit of the DC machines to DC Supply #1 (M-5 & M-6).

4. Connect the armature circuit of the DC machine and the field circuit of the AC machine to DC Supply #2 (M-7 & M-8).

5. Turn on DC Supply #1 (DC1) and adjust it to 100 volts (100Vdc).

6. Close the field circuit switch on the DC machine and adjust the field current to approximately 1.5 amps (1.5 Adc). (The internal resistance is approximately 64 ohms, so adjusting each of the 4 rheostats in the field circuit to 0 ohms results in approximately 1.5 amps.)

7. Start the DC Machine by turning on DC Supply #2 and bringing its voltage smoothly up to 115 volts.

8. Monitor the AC voltage output of the Synchronous AC Machine on the oscilloscope using “Line” synchronization and adjust the rheostats in the DC Machine field circuit to supply enough power to bring the AC machine to synchronous speed, the speed at which the AC voltage stands still on the scope. Use the "Frequency" measurement function on the scope to also observe the frequency. Also, observe the motor speed using the strobe light to insure synchronous speed operation.

AC Generator Operation - Demonstration Procedure Step 1.

1. Read and record the AC Machine output voltage (line-to-neutral) with the field circuit switch of the AC machine open.

2. Close the switch in the AC Machine field circuit and adjust the field current 0.0 (0.5) 3.5A using the rheostats in the AC machine field circuit to obtain different AC output voltages. Adjust as necessary the DC Machine field current to maintain synchronous speed for the AC machine.

3. Read and record the various parameters for the DC and AC Machines as required by Step 1 of the laboratory procedure.

4. Adjust the AC Machine field current to a minimum and open the field circuit switch.

DC Motor Shutdown Procedure

1. Adjust DC Machine field current to a maximum.

2. Smoothly adjust the DC Supply #2 voltage to a minimum to stop the DC motor and shut off DC Supply #2.

3. Shut off the DC Supply #1. Set DC Supply #1 voltage to 100 volts if you plan to run the AC machine as a motor as the next step. If you do not, then adjust the DC Supply #1 voltage to 0 volts before turning it off.)

4. Open the DC Machine field current switch.

Demonstration Step 2 - AC Machine Equivalent Reactance (X)

1. Short secondary of current transformer to protect current meter from startup surge. Turn on the AC supply, and raise the voltage to 175V (line-to-line). With zero field current, the AC machine will be a poor induction motor. With the DC machine providing virtually no load, the AC machine will rotate, but its speed may be far below synchronous speed. Un-short the current transformer

3. Take readings per Step 2 of the laboratory procedure (i.e. phase voltage, line current, phase power)

4. Lower the AC supply voltage to 0V and shut off the AC supply.

Demonstration Step 3 - AC Motor Driving DC Generator with Resistive Load

AC Motor Start–up Procedure

1. DC Supply #1 may be set to a minimum or 100 volts, but it must be OFF. Insure also that DC Supply #2 is OFF. Insure that the AC supply is set to the minimum voltage and that it is OFF.

2. Insure that the switches in the field circuits of the two machines are both OFF.

3. Insure that the armature of the DC Machine is disconnected from DC Source #2 and connected to the six parallel rheostats forming a 6 ohm load capable of 20 Amps (2400 Watts).

4. Adjust the rheostats in the field circuit of the AC machine to maximum resistance.

5. Adjust DC Supply #1 to 100 volts and DC Supply #2 to 100 volts.

6. Short the current transformer output coil and close the field circuit switch for the AC Machine only and adjust the current to a minimum value of approximately 0.5 Amps.

7. Turn on the AC Supply and insure it is at minimum voltage and 0 current.

8. Smoothly adjust the AC line-to-line voltage to 208 Volts rms to start the AC motor. Monitor the current to see that it does not exceed 40 Amps rms. It should settle to less than 30 Amps rms at 208 Volts rms.

9. Quickly adjust the AC Machine field current to approximately 1.5 Amp to obtain synchronous speed. You can tell by the sound of the AC Machine that it is running in synch with the 60 Hz AC input.

AC Motor Operation - Demonstration Procedure Step 3

1. Adjust the series rheostats in the field circuit of the DC Machine to a maximum and close the switch in the field circuit of the DC Machine to cause it to generate a controllable DC voltage. Adjust the series connected rheostats in the field circuit of the DC machine as necessary to obtain and/or maintain the constant AC Machine input power (200, 450, and 700 Watts per phase) required for performance of the experiment. Note that this actually controls the DC Machine output voltage and, therefore, the power supplied to the six parallel rheostats forming the 6 ohm load. Adjust phase voltage to 120 V at IA min.

2. Adjust the field current 0.0 (0.5) 3.5 A of the AC Machine to obtain “Vee Curve” readings. The field current of the DC machine will also have to be adjusted each time to maintain the desired AC Machine input power.

3. Read and record the various parameters of the AC Machine as required by Step 3 of the laboratory procedure. Work quickly when running at 700 Watts input power per phase to the AC Machine as this causes large currents (approximately 20 Amp) to be supplied by the DC Machine to the 6 ohm load.

Shutdown Procedure

1. Adjust the field current of the DC Machine to a minimum value to minimize the AC machine input power and then open the switch in the field circuit of the DC Machine.

2. Adjust the field current of the AC Machine to approximately 0.5 Amp while insuring it remains in synchronous operation.

3. Smoothly adjust the AC supply to minimum voltage to bring the AC Machine to a stop and then shut off the AC supply and the AC panel switch connecting the AC supply to the Motor Room.

4. Adjust the field current of the AC Machine to a minimum and open the switch in the field circuit of the AC Machine.

5. Adjust both DC Supplies to minimum and shut off both DC Supplies.

Special "tricks" which may prove useful are:

(a) Construct a 6-Ω/20-A load for the DC machine by paralleling six 36-Ω resistors. Because the 50-Ω variable rheostats are rated at 4.5 A, do not linger at higher currents.

(b) In Part 1, use a one DC supply for separate excitation of the shunt winding of the DC machine and a different DC supply to drive the DC machine’s armature.

(c) For the purposes of determining phase power and current, use a 20-A:5-A current transformer to drive the 5-A terminal of the YEW wattmeter.

(d) To control each of the two DC field currents, use four 50-Ω rheostats in series. Both field windings should be powered from the same DC supply.

CAUTION: Due to high output voltage, the synchronous generator should not be connected directly to a frequency counter or an oscilloscope. To observe the generator waveform and measure rpm, connect a 30:1 step-down isolation transformer to one phase of the synchronous machine plug and then examine the much lower output of the grounded secondary.

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Year by year, your faculty attempt to improve the Vee-curves that are obtained. Current thinking is that the synchronous machine is not a well-balanced load and that therefore the neutral return[pic]!,1CFLMUVW\]^_cdeflmnrstz‰Š-$)67†

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